Please refer to FIG. 1. FIG. 1 illustrates a perspective view of a foldable display device 20a according to prior art. The conventional foldable display device 20a comprises a first plate member 500a and a second plate member 600a pivotally connected to each other and capable of being folded and spread. A flexible display member 700a is disposed on a front face of the first plate member 500a and the second plate member 600a. When the first plate member 500a and the second plate member 600a are folded relatively, the flexible display member 700a completely attaches to a front face 501 of the first plate member 500a and a front face 601 of the second plate member 600a, and a middle portion 730a of the flexible display member 700a attaches to a side 502 of the first plate member 500a and a side 602 of the second plate member 600a. However, when the first plate member 500a and the second plate member 600a are spread relatively to form a surface, the sides 502, 602 of the first plate member 500a and the second plate member 600a close to each other become closer than their previous posture. It causes the middle portion 730a of the flexible display member 700a (i.e., the portion attaching to the sides 502, 602 of the first plate member 500a and the second plate member 600a when the first plate member 500a and the second plate member 600a are folded relatively) to hump such that the flexible display member 700a forms an uneven surface. As a result, the experience of use is bad, the image displaying is uneven, and the lifespan of the flexible display member is shortened.
In addition, a kind of hinge with four gears is disclosed in published US patent applications with publication nos. 2012/0096678, 2011/0271486, 2012/0042473, 2013/0139355, and 2015/0345195. These hinges can be applied to portable devices; however, these hinges cannot be applied to flexible displays. The issue that a middle portion between two plate members (i.e., a base and a cover) of the flexible display humps when the two plate members are spread to form a surface is still not solved. Another driving device between two main axles of a hinge is disclosed in published US patent applications with publication nos. 2015/0362958 and 2015/0267450; however, it does not solve the issue of humping of the flexible display.